Wells of the type commonly used for petroleum or geothermal exploration are often on the order of 1.5 miles deep. Typically, these wells or “boreholes” are drilled using pipes (also referred to as “drill strings”) assembled from relatively light metal sections connected end-to-end by tooling joints. These sections are generally about 30 to 45 feet long. To form a borehole, the drill string is rotated such that a drill bit attached to its “downhole” or operative end bites into the earth. Additional sections are typically added to the “uphole” or surface end of the drill string as the borehole deepens.
In addition, a fluid, often referred to as “drilling mud” can be pumped through an axial hole in the drill string from the surface to the downhole end of the drill string. The drilling mud exits the drill string at the downhole end and returns to the surface through the space between the drill string and the borehole. The drilling mud serves several purposes, including, for example, cooling and lubricating the drill bit, powering the drill bit through hydrodynamic pressure, providing a deposit on the borehole wall to seal the formation, and removing loose cuttings and debris from the borehole.
When exploring and/or recovering natural resources by drilling in this manner, real-time communication between the downhole unit disposed proximate to the end of the borehole and the surface unit disposed near the opening of the borehole is desirable. For example, it enables an operator of the drill string to monitor one or more sensors located proximate to the operative end of the drill string, and to steer the drill bit depending on data received from the sensors. Communication from the downhole unit to the surface unit of data relating to, for example, the temperature and/or pressure of the drilling mud proximate to the drill bit may also be desirable. In addition, it may be desirable to transmit management information from the downhole unit to the surface unit or, alternatively, in the opposite direction. Exemplary management information includes additional information relating to the sensed data, such as, for example, how much and what type of data has been sensed, when the transmission of the sensed data to begin, and, if different types of data have been sensed, the order in which they are to be transmitted.
One conventional data communication or “telemetry” technique is often referred to as a “mud pulse,” whereby the borehole is filled with the artificial drilling mud, whose density, viscosity, and other properties are adjustable according to the condition of the formation drilled. A valve located at the drill bit is cycled in accordance with a data sequence so that each actuation of the valve releases a pressure pulse from the drill string into the fluid-filled borehole. This technique, however, has several drawbacks. For instance, data communication is only unidirectional, i.e., from the downhole unit to the surface unit, and the data rate is less than 1 bit/second. In addition, the mud pulse technique requires a pressurized drill string, which limits the drill string's applicability. Poor drilling performance may also result.
Other techniques for communicating between the downhole unit and the surface unit have also been employed, including electromagnetic radiation through the ground formation and electrical transmission through an insulated conductor. Both of these methods have, however, disadvantages associated with signal attenuation, ambient noise, high temperatures, and compatibility with standard drilling procedures. For example, known electromagnetic telemetry systems deliver data at a rate of about only 1 bit/second out to 6000 feet.
Some researchers have also pursued drill string telemetry by acoustic wave propagation through a metal drill string. One technique, for example, relies on narrowband frequency shift key modulation. Disadvantages of this approach, however, include a data rate that is limited to 8 bits/second and a communication range that is effectively limited to 6000 feet. In addition, these acoustic telemetry systems do not operate while drilling is in progress. When they do operate, communication is only unidirectional. Another disadvantage is that characterization and sorting of tubing is often required.
Another similar approach utilizes narrowband amplitude key modulation. Disadvantages of this approach also include a data rate that is limited to 8 bits/second and a communication range that is effectively limited to 6000 feet. A telemetry system based on narrowband amplitude key modulation also does not operate while drilling is in progress, and, when it does operate, communication is only unidirectional. Another disadvantage is that calibration is required prior to each transmission.
There is, therefore, a need for a versatile acoustic telemetry system that addresses the shortcomings of prior approaches.